Recent advances in superconductivity have led to an increased interest in the development and commercialization of superconducting electromechanical rotating (SER) devices such as large electric generators and large electric motors, including synchronous AC motors. Such devices typically include a superconductive rotor having a vacuum jacket and a stator coaxially surrounding the rotor. The superconducting windings are disposed inside of the vacuum jacket on a winding support structure. The winding support structure and windings are cooled to a cryogenic temperature. One such device is a high temperature superconducting (HTS) electromechanical device which uses a HTS winding in the rotor of the device rather than a low temperature superconducting winding. In the case of a synchronous AC motor, the stator and rotor of the typical SER device are configured such that the rotor is rotated synchronously with the rotating stator magnetic field.
The superconducting synchronous motors generally have air-core geometry meaning that a significant part of magnetic flux passes through non-ferromagnetic materials. It may be because the stator core does not have teeth, the rotor does not have ferromagnetic pole shoes or does not include ferromagnetic materials at all, and so on. Such machines have large air gap, which possess problems with end-winding and core end region eddy current losses due to a much higher than normal leakage fields. Axial fluxes caused by currents flowing in the rotor and stator end windings are sufficiently great to induce significant eddy currents in laminations at each end of a stator core, in core clamping plates and motor frame. The circumferential/radial eddy currents generate high losses in the motors.
Various methods used to minimize eddy current losses in the core end regions are: (1) conducting screens on core end plates to act as flux diverters; (2) profiling an end of the core, e.g., locally increasing the reluctance of the rotor/stator gap; (3) segmentation of the laminations; (4) using narrow slits—“pistoye slots”—in rotor teeth to lengthen a path taken by the eddy currents, thereby increasing path resistance and decreasing current/losses; and (5) using extra coatings of insulating varnish on the laminations. Thus, core end region design is conventionally employed as a compromise between keeping the eddy current losses small yet maintaining adequate magnetic, thermal and mechanical properties.